A tsunami's time of danger can last for several hours after the first wave has hit the shore. A tsunami is a series of waves or surges that can hit at intervals of 5 minutes to 1 hour. There is no direct method to predicting the precise amount of time that a tsunami poses a threat to people in the area, as the surges make it appear as if the ocean water is retreating before advancing again upon the shore.
Tsunamis are not always detectable at sea, because of the immense depth of the ocean. A tsunami that is only a few inches above the sea level on the open ocean can quickly become a 30-foot wave hitting the shore. This is due to the fact that the wave travels very quickly, around 500 mph along the surface of the ocean, and then hits the shallow waters nearby shores, where the wave slows along the bottom level of the water but maintains its speed along the top level of water. This causes a dramatic and often devastating rise in the sea level along a shoreline.
When a tsunami reaches the shoreline, it can be as small as a few inches tall, which can still cause significant damage depending on the location and structures around the area, or it can be as tall as 100 feet. The average sea level rise from a tsunami is 10 feet.